Electrodeless lamp igniter system

ABSTRACT

An electrodeless lamp resonant cavity apparatus for use as a spectral source in atomic absorption spectroscopy. The apparatus utilizes a cavity resonator to excite the lamp. A tuning means is utilized to ignite the lamp by momentarily sweeping the tuning of the cavity with the unexcited lamp through the most favorable condition for ignition. Upon release the tuning means returns the tuning of the cavity to that condition most favorable to maintaining the lamp discharge.

United States Patent [191 Hruda et al.

[ ELECTRODELESS LAMP IGNITER SYSTEM [75] Inventors: Robert M. Hruda;Howard E. De Haven, both of Horseheads, NY.

[73] Assignee: Westinghouse Electric Corporation,

3/l939 Bcthcnod 3l5/248 July 30, 1974 7 Primary ExaminerHerman KarlSaalbach Assistant Examiner-James B. Mullins Attorney, Agent, or Firm-W. G sutcliff [5 7 ABSTRACT An eiectrodeless lamp resonant cavityapparatus for use as a spectral source in atomic absorptionspectroscopy. The apparatus utilizes a cavity resonator to excite thelamp. A tuning means is utilized to ignite the lamp by momentarilysweeping the tuning of the cavity with the unexcited lamp through themost favorable condition for ignition. Upon release the tuning meansreturns the tuning of the cavity to that condition most favorable tomaintaining the lamp discharge.

5 Claims, 3 Drawing Figures PATENTEU JUL30|974 3. 826.950

FIG. 3

ELECTRODELESS LAMP IGNITER SYSTEM BACKGROUND OF THE INVENTION Thisinvention relates to a radio frequency excited light source, and moreparticularly, to a source capable of emission of light of a wavelengthcharacteristic of a material within a lamp member, and primarily forutilization in atomic absorption spectroscopy. In this type of source,it is necessary to use a gaseous type discharge lamp wherein an are orplasma is created in a vapor which will, when electrically excited tooptical emission, produce one or more spectral lines characteristic ofthe vapor material. The material may be a gas, metal or a metallic salt.One type of light source utilized in this area is the hollow cathodetype, such as described in US. Pat. No. 3,264,511 issued Aug. 2, 1966 toG. K. Yamasaki and assigned to the same assignee as this invention. Thehollow cathode lamp is the most common type of spectral source.

There also has been a considerable amount of work on electrodelessdischarge lamps as spectral sources.

The term electrodeless means that the lamps are provided withoutelectrodes within the envelope, and the discharge is initiated andmaintained by radio frequency electric fields. The radio frequency maybe in range of 300 to 3,000 megahertz. These types of lamps providehigher intensity as compared to the hollow cathode lamps and freedomfrom contamination due to the presence of electrodes and other elementswithin the envelope. The electrodeless lamp generally comprises acylindrical quartz envelope whose length is normally greater than itsdiameter. The volume within the envelope in most applications is filledto a few torrs with a suitable gas such as argon or neon plus avaporizable salt of the desired" element. The lamp is then mounted in asuitable radio frequency field region such as that generated by thecavity resonator, an antenna or some other means. Upon application of aradio frequency field to the lamp proper, the gas, such as argon, isionized. This discharge tends to heat the quartz envelope and the saltsuch as lead iodide, until enough of the lead is vaporized, at whichtime the lamp switches over from running on argon to running onvaporized salt. The result is the desired spectral emission due to lead.These sputtered atomic particles of the lead are bombarded and a portionare excited from a ground state to a higher energy level. When theseexcited sputtered atoms return to their lower ground state level, energyoriginally absorbed from the bombarding particles is released in theform of radiation having spectral lines characteristic of the lead inthe envelope. This beam of spectral radiation is then directed through avaporized sample of a solution or material to be analyzed. If thewavelength of radiation from the spectral source corresponds to thatrequired to excite the vaporized atoms in the sample which are at groundstate, a portion of the intensity of the spectral radiation will beabsorbed by the atoms of the vaporized sample solution. The normalpractice is then to direct the radiation after transmission through theunknown sample to a monochromator which is adjusted to the wavelength ofthe initial radiation from the source. A suitable detector is associatedtherewith which measures the amount of absorption of that passingthrough the sample and that not passing through the sample, so as toarrive at a determination of the amount of spectral radiation absorbedwithin the unknown sample. This atomic absorption technique is wellknown in the art.

In the radio frequency discharge type light source, the lamp proper orenvelope is mounted within the high electric field region of a cavity.The purpose of the cavity is to transfer power from a radio frequencysource to the gas within the lamp proper. The resonant cavity structureis utilized to increase the electric field found within the cavity. Itis, of course, desirable to match the impedance of the cavity to that ofthe coaxial line of the radio frequency power supply. When the resonantfrequency of the cavity is tuned to that of the radio frequency sourceand the impedance is matched, then power reflected from the cavity is ata minimum.

It is found that the design of such a resonant cavity for excitation ofelectrodeless lamps is complicated because the lamp represents twodistinct radio frequency loads. When extinguished, the lamp is a simplecapacitance, while when ignited, the lamp is equivalent to a complexreactance including inductance, resistance and capacitance. If thecavity is tuned to the best operating frequency of the lamp, seriousdetuning occurs if the lamp is extinguished and reignition is attempted.The disadvantage of a resonant cavity is that the reactivecharacteristics of, the discharge varies with the state of the dischargein the lamp. The spectral radiation of the discharge is extremelysensitive either to changes in incident radio frequency power or tochanges in the coupling efficiency between the'radio frequency sourceand the discharge. One solution is to provide a triggering voltage inthe form of a radio frequency field to produce the initial dischargewhich is thereafter sustained by the radio frequency energy introducedin the cavity. Such a solution, of course, results in additionalcircuitry and voltage supplies.

SUMMARY OF THE INVENTION This invention is directed to a radio frequencyexcited light source utilizing a cavity resonator in which apparatus isprovided for tuning of the cavity to the most favorable condition formaintaining a discharge within the light source and providing additionaltuning apparatus to momentarily sweep the tuning of the cavity throughthe most favorable condition'for igniting the discharge within the lightsource.

BRIEF DESCRIPTION OF THE DRAWINGS For better understanding of theinvention, reference may be had to the preferred embodiments, exemplaryof the invention, shown in the accompanying drawings in which:

FIG. 1 is a longitudinal sectional diagram of a radio frequency excitedlamp apparatus incorporating the teachings of this invention;

FIG. 2 is an end view of the cavity apparatus illustrated in FIG. 1; and

FIG. 3 is an enlarged sectional view of the tuning means embodied inFIGS. 1 and 2.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1, there isillustrated a cavity resonator 9 comprised of a cylindrical member 10which may be of a suitable electrical conductive material or at leasthaving an electrical conductive coating on the inner surface thereof ofa material such as silver. The length of the cylindrical member 10 maybe about 10 cm. The diameter of the cylindrical member may be about 4cm. Positioned at one end of the cylindrical member 10 is an end plate12 with an opening 14 therein through which radio frequency energy iscoupled into cavity 11 defined by the member 10 from a radio frequencysource 16. A window 33 such as glass may be provided on the opposite endof the member 10. The window 33 istransmissive to radiation from thelamp 24. An opening may be also provided in the window 33 or the window33 may be omitted. A matching network 18 may be provided between thesource 16 and the cavity 11 to provide optimum transfer of energybetween the source 16 and the cavity 11. A loop 20 may be providedwithin the cavity 11 to couple the energy into the cavity 11. A tubularmember 22 is secured at one end to the end plate 12 and extends for adistance of about 6 cm. therefrom. The diameter of the member 22 may beabout 1.5 cm. Positioned near the end of the member 22 is an envelope orlamp 24 which contains a gas and an element whose characteristicemission is desired. The lamp 24 may have a diameter of about 10millimeters and a length of from less than 1 centimeter to afewcentimeters. The volume within the envelope 24 may be a few cubiccentimeters. The envelope 24 may have a specially defined window 25 fortransmission of the spectral radiation.

A graphite sleeve may be provided about the envelope 24 and the envelopeis located within the opening in the tubular member 22. The envelope mayproject out of the member 22 for a short distance about /2 centimeter.The electric field in the cavity 11 is concentrated at the nose of themember 22.

A tuning means is associated with the cavity resonator' 10. The tuningmeans 30 includes a main tuning screw or sleeve 38 which is insertedinto the cavity 11, and its extension into the cavity 11 may be adjustedby means of the thread on the outer surface engaged a threaded opening39 in the wall 10. The tuning means 30 also includes a capacitor plate32 within the cavity 10 which'is mounted on a push-button shaft 34. Theplate 32 as well as sleeve 38 should be of electrically conductivematerial or coated therewith. The shaft 34 is positioned within acentral region of the tuning sleeve 38 and a spring member 36 ispositioned therein to bias the shaft 34 outwardly in a relaxed orstationary condition as indicated by the dotted lines for the platemember 32. Pushing the shaft 34 inwardly moves the capacitor plate 32inwardly, as indicated in FIG. 3, to a temporary position such asillustrated.

Tuning means 30 is adjusted in the relaxed or stationary condition byadjusting the sleeve 38 such that the cavity 10 is tuned to near theresonant frequency with the electric discharge lamp 24 ignited toprovide high stability of light output. This will provide a large amountof electric field in the discharge portion of the envelope 24. In theunexcited condition, the cavity resonator 9 will not be tuned to nearresonance; but by pushing the push-button shaft 34 inwardly, thecapacitor plate is thrust into the cavity 11, thereby passing the cavityresonator 9 through a condition of resonance with regard to theunexcited condition. As a result, a

high electric field is generated in the electric discharge region oflamp 24 and the lamp 24 is ignited by the high field produced. The shaft24 is then released, and the cavity reverts to its condition of maximumefficiency with the activated lamp.

It is, of course, obvious that there may be other modifications to theinvention, such as different tuning means associated with the cavity andalso, if desired, the ignition system could be distinct from the maintuning member.

We claim as our invention:

1. A radio frequency light source comprising means defining a cavityresonator, an electrodeless discharge lamp envelope positioned withinsaid cavity resonator, said cavity resonator having an opening throughwhich the light generated by said discharge lamp may pass to theexterior of the cavity, means for introducing radio frequency power intosaid cavity resonator for producing an electric field in the region ofsaid discharge lamp, a first tuning means for said cavity resonator fortuning said cavity resonator to a frequency to provide a stable spectraloutput from said lamp in an excited condition and a second tuning meansfor momentarily tuning said cavity resonant through a favorablecondition for ignition of said discharge lamp.

2. The light source set forth in claim 1, in which said first tuningmeans is adjustable to establish a substantially stationary position tomaintain a favorable tuning for a stable light output from said lamp inthe excited condition and said second tuning means'is movable tomomentarily sweep through a plurality of positions to substantiallymaximize tuning for favorable ignition of said lamp.

3. The device as set forth in claim 2, in which said firstand secondtuning means have a common tuner member and said second tuning means isa spring loaded shaft member, such that said spring biases said tuningmember in said stationary position and said shaft member position ismanually operable to move said tuning member through a plurality oftemporary positions to thereby sweep the tuning of said cavity through afavorable temporary position for excitation of said lamp and then returnto said stationary position favorable to maintaining said discharge.

4. A radio frequency light source comprising a cavity resonator, anelectrodeless discharge lamp containing a material of which itscharacteristic spectral radiation is desired, said cavity resonatorhaving a transmissive portion through which said characteristicradiation may pass, means for establishing an electric field within saidcavity with said lamp positioned within a region of high electric field,tuning means provided within said cavity resonator to momentarily sweepthe tuning of said cavity resonator to maximize tuning of said cavityresonator for favorable ignition of said lamp.

5. The light source set forth in claim 4 in which said tuning means isreturned to a stationary position after said sweep, said stationaryposition providing a favorable electric field for maintaining a stableemission from said lamp.

1. A radio frequency light source comprising means defining a cavityresonator, an electrodeless discharge lamp envelope positioned withinsaid cavity resonator, said cavity resonator having an opening throughwhich the light generated by said discharge lamp may pass to theexterior of the cavity, means for introducing radio frequency power intosaid cavity resonator for producing an electric field in the region ofsaid discharge lamp, a first tuning means for said cavity resonator fortuning said cavity resonator to a frequency to provide a stable spectraloutput from sAid lamp in an excited condition and a second tuning meansfor momentarily tuning said cavity resonant through a favorablecondition for ignition of said discharge lamp.
 2. The light source setforth in claim 1, in which said first tuning means is adjustable toestablish a substantially stationary position to maintain a favorabletuning for a stable light output from said lamp in the excited conditionand said second tuning means is movable to momentarily sweep through aplurality of positions to substantially maximize tuning for favorableignition of said lamp.
 3. The device as set forth in claim 2, in whichsaid first and second tuning means have a common tuner member and saidsecond tuning means is a spring loaded shaft member, such that saidspring biases said tuning member in said stationary position and saidshaft member position is manually operable to move said tuning memberthrough a plurality of temporary positions to thereby sweep the tuningof said cavity through a favorable temporary position for excitation ofsaid lamp and then return to said stationary position favorable tomaintaining said discharge.
 4. A radio frequency light source comprisinga cavity resonator, an electrodeless discharge lamp containing amaterial of which its characteristic spectral radiation is desired, saidcavity resonator having a transmissive portion through which saidcharacteristic radiation may pass, means for establishing an electricfield within said cavity with said lamp positioned within a region ofhigh electric field, tuning means provided within said cavity resonatorto momentarily sweep the tuning of said cavity resonator to maximizetuning of said cavity resonator for favorable ignition of said lamp. 5.The light source set forth in claim 4 in which said tuning means isreturned to a stationary position after said sweep, said stationaryposition providing a favorable electric field for maintaining a stableemission from said lamp.